The present invention relates to a magnetic recording medium and a magnetic storage system which are capable of recording a large amount of information and, more particularly, to a magnetic recording medium suitable for high-density magnetic recording and a magnetic storage system in which this magnetic recording medium is used.
Needs for large-capacity design of magnetic storage systems are growing more and more. In recent years, to meet the needs, it has rapidly proceeded to adopt a composite type head in which a writing head and a reproducing head are separately provided and in which a highly sensitive magneto-resistive head is used as the reproducing head. Further, a more highly sensitive head has also been put to practical use. In this more highly sensitive head, in order to increase the sensitivity of a magneto-resistive head that utilizes a change in the electric resistance of elements caused by a change in the leakage flux from a medium, there is utilized a very large magnetoresistance change (giant magnetic resistance effect or spin-valve effect) occurring in a plurality of magnetic films layered via a non-magnetic film. This head utilizes the phenomenon that the relative magnetization direction of a plurality of magnetic films layered via the non-magnetic film is changed by a leakage magnetic field, resulting in a change in magnetoresistance.
On the other hand, as requirements for high-density design of magnetic recording mediums, a noise reduction and a reduction in a regenerative output change occurring with the lapse of time due to thermal fluctuations are necessary.
In order to realize a recording density exceeding 20 Gb/inch2 in a magnetic disk medium, it is necessary to increase coercivity and besides to reduce the unit of magnetization reversal occurring in a magnetic film. For this purpose, it is necessary to make magnetic grains fine in size which constitute the magnetic film.
JP-B-2674131 discloses a thin magnetic film which is expressed by a composition equation (Coa Ptb Bc Mxc3xa4)100xe2x88x92x Ox (where a, b, c, xcex4, x are atomic %), and has composition ranges of:
0 less than a=100xe2x88x92bxe2x88x92cxe2x88x92xcex4;
0 less than bxe2x89xa650;
0.1xe2x89xa6c bxe2x89xa630;
0 less than xcex4xe2x89xa630;
and
0xe2x89xa6xxe2x89xa615
where, the above M is at least one kind selected from the group consisting of Ti, Zr, V, Cr, Nb, Mo, Ta, W, Fe, Ni, Si, Al, Ge, Ga, In, Sn, Pb, Sb, Bi, P, Se, C, Zn, Cu, Ag, Au, Ru, Pd and Re.
As an example in which this thin magnetic film is applied to a magnetic recording medium, JP-A-2-73511 discloses a CoPt-base alloy or CoPto-base alloy magnetic recording medium which has a thin magnetic film containing B or at least one kind selected from the following group of elements MI, wherein the above thin magnetic film is formed on an underlayer which contains at least one kind selected from the following group of elements MII and has a thickness of 1 to 1xc3x97104 nm (1xc3x9710 to 1xc3x97105 xc3x85). The thin magnetic film contains, as the above elements MI, at least one kind selected from the group consisting of Ti, Zr, V, Cr, Nb, Mo, Ta and W and, the underlayer contains, as the above elements MII, at least one kind selected from the group consisting of Ti, Zr, V, Cr, Nb, Mo, Ta, W, Hf, Mn, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, In, Ti, C, Si, Ge, Sn, Pd, P, As, Sb, Bi, S, Se, Te, Be, Mg, Ca, Sr, Ba, Sc, Y and rare-earth elements.
Further, there is proposed in JP-A-60-228637 a Co-base alloy for magnetic recording medium""s which contains Cr in an amount of 9.0 to 22.5 wt., %, at least one kind selected from the group consisting of Sc, V, Nb, Ta, W, Mn, Tc, Re, Fe, Os, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ca, In, Tl, C, Si, Ge, Sn, Pb, P, As, Sb, Bi, S, Se, Te, lanthanoid elements and actinoid elements in an amount of 0.001 to 5 wt. % in total, and the balance Co.
The technology described in the above JP-B-2674131 relates to a perpendicular magnetic recording medium and was unsuitable for longitudinal magnetic recording because of a larger perpendicular coercivity than a longitudinal coercivity. Further, in the conventional technology described in JP-A-2-73511, there is proposed a medium structure for improving perpendicular coercivity, longitudinal coercivity or perpendicular magnetic anisotropy. However, when a magnetic recording medium was formed by optionally selecting Cr and Ru, respectively, from the above element groups MI and MII, it was difficult to reduce medium noise and hence it was necessary to further reduce the noise. Further, there was a limit to the control of the crystal grain size of a magnetic film constituting the above magnetic disk medium, and fine grains and coarse ones coexisted in the magnetic film. Such magnetic films were affected by a leakage magnetic field from surrounding magnetic grains in information recording and, inversely, coarse magnetic grains gave interactions, with the result that stable signal regeneration was sometimes impossible in high-density recording with a recording density exceeding 20 Gb/inch2.
The technology described in the above JP-A-60-28637 relates to a perpendicular magnetic recording medium and it was necessary to adjust saturation magnetization to not more than 8.5xc3x9710xe2x88x922 T (850 gauss) which is a practical range. Saturation magnetization of not more than 8.5xc3x9710xe2x88x922 T (850 gauss) was too small and this perpendicular magnetic recording medium was inappropriate for a material composition for longitudinal magnetic recording.
Further, the present inventors have found out that making the distribution of magnetic grain size uniform, along with making magnetic grains fine in size, is important for a reduction in thermal fluctuations.
Therefore, the first object of the invention is to provide a low-noise, high-performance magnetic recording medium by making magnetic grains fine in size through the optimization of the composition of a magnetic film.
The second object of the invention is to provide a magnetic recording medium of low thermal demagnetization by controlling the distribution of the magnetic grain size in a small range.
The third object of the invention is to provide a large-capacity magnetic storage system capable of high-density recording with a recording density exceeding 20 Gb/inch2.
In the magnetic recording medium having a recording film formed of at least one magnetic layer formed on a substrate via an underlayer, the above first object is achieved when the above at least one magnetic layer is formed of an alloy containing cobalt as the main constituent thereof, chromium and germanium as the additives of the alloy and containing boron or phosphorus as the third additive of the alloy. The above first object is more readily achieved by use of layered structure having at least two underlayers as the above underlayer. Furthermore, when the residual magnetic flux density of the above recording film measured while applying a magnetic field in the relative traveling direction of a magnetic head during recording to the recording medium is expressed as Br while the thickness of the above recording film is expressed as xe2x80x9ctxe2x80x9d, it is preferred that the value of the product Brxc3x97t be not less than 1.6 mA but not more than 5 mA. Here, in a case where the recording film is formed of a plurality of magnetic layers, xe2x80x9ctxe2x80x9d is the total of the thicknesses of all magnetic layers constituting the recording film. Further, in a case where the magnetic recording medium is a magnetic disk medium used in a magnetic disk device, the above expression xe2x80x9crelative traveling direction of a magnetic head during recording to the recording mediumxe2x80x9d means the circumferential direction of the magnetic disk.
Further, it is preferred that, when the third additive is boron, the contents of the boron and germanium both contained in the above at least one magnetic layer be not less than 1 but not more than 14 atomic % and not less than 5 but not more than 22 atomic %, respectively. In this case it is preferred that the content of the chromium contained in the above at least one magnetic layer be not less than 15 but not more than 28 atomic %. When these additives are simultaneously contained, the first and second objects of the invention are simultaneously achieved by fabricating a magnetic recording medium in which it is preferred that the total of the boron and chromium contents is not less than 20 but not more than 32 atomic %.
When the at least one magnetic layer contains platinum as the fourth additive, it is preferred that the content of platinum contained in the at least one magnetic layer formed on a substrate via an underlayer is not less than 2 but not more than 14 atomic % and that the total content of germanium and platinum is not less than 17 but not more than 24 atomic %.
When the third additive is phosphorus, it is preferred that the contents of phosphorus and chronium both contained in the at least one magnetic layer be not less than 1 but not more than 12 atomic % and not less than 15 but not more than 28 atomic %, respectively. The first and second objects of the invention are simultaneously achieved by forming a magnetic recording medium in which the total of the phosphorus and chromium contents is not less than 20 but not more than 32 atomic %. When the at least one magnetic layer contains platinum as the fourth additive, the content of the platinum contained in the magnetic layer is preferably not less than 2 but not more than 14 atomic %.
In a magnetic storage system comprising: the above magnetic recording medium; a drive portion for driving the magnetic recording medium, a magnetic head provided with a writing part and a regenerating part; means for relatively moving the magnetic head with respect to the magnetic recording medium, a mechanical part for ramping the magnetic head; means for inputting signals into the magnetic head; and recording/regenerating signals-processing means for regenerating output signals transferred from the magnetic head, the regenerating part of the magnetic head comprises a magneto-resistive sensor which includes a plurality of electrically conductive, magnetic films in which a large resistance change occurs in response to a relative change of magnetization direction which relative change is caused by an external magnetic field, and a magneto resistive sensor including an electrically conductive, non-magnetic film located between the electrically conductive, magnetic films and, at the same time, the magnetic recording medium is constituted by the magnetic recording medium specifically explained above. As a result of this, the magnetic storage system described in the third object of the invention is obtained.
It is preferred that the above substrate be a rigid substrate, such as a chemically-reinforced substrate with a diameter of 65 mm. However, the diameter of the disk may be 95 mm, for example, and is not limited to 65 mm. The material for the substrate may be made of Ti, etc. if it permits high-speed rotation, and is not limited to glass.
In forming a magnetic recording medium in which a magnetic layer is formed on a substrate via an underlayer, the underlayer may be formed as only one layer of underlayer of Crxe2x80x94Ti alloy of body-centered cubic structure, etc. However, after the forming of an initially-grown layer of Crxe2x80x94Ti alloy (a first underlayer), a second underlayer made of another Cr alloy, such as a Crxe2x80x94Mo alloy, different from the initially-grown layer may be formed. In order to ensure that the Cr alloy forms an interface capable of hetero-epitaxial growth with a thin magnetic film of the Co alloy formed thereafter, it is possible to coordinate the lattice constant by adjusting the content of the additives. As a result of this, it becomes possible to form on the Cr alloy underlayer film of body-centered cubic structure the Co alloy magnetic film, in which the c-axis of hexagonal closest packing structure is oriented in parallel with the surface of the film, enabling a medium suitable for longitudinal magnetic recording to be formed. Alternatively, after the formation of a Coxe2x80x94Crxe2x80x94Zr alloy film as a first bonding layer for improving bonding with the substrate, an Nixe2x80x94Crxe2x80x94Zr alloy film may be formed, and after surface adjustment thereof, an underlayer made of the Cr alloy and the magnetic layer maybe formed in this order.
As mentioned above, the above first object is achieved when the at least one magnetic layer is formed of the alloy containing cobalt as the main constituent thereof and containing boron, chromium, germanium and platinum or containing phosphorus, chromium, and germanium as the additives of the alloy, because the addition of boron or phosphorous promotes, the fining of the size of grains, because the addition of chromium accelerates the decrease in magnetic coupling, enabling noise in the medium to be reduced, and because the addition of germanium reduces thermal fluctuations and simultaneously and promotes the segregation of chromium, resulting in noise reduction, all these factors producing a synergistic effect.
As mentioned above, when the residual magnetic flux density of the above recording film measured by applying the magnetic field in the relative traveling direction of the magnetic head during recording relative to the recording medium is expressed as Br and the thickness of the above recording film is expressed as t, it is preferred that the value of the product Brxc3x97t be not less than 1.6 mA but not more than 5 mA, because of the reason explained below. When the value of Brxc3x97t is smaller than 1.6 mA, the regenerative output becomes too small and simultaneously it becomes difficult to reduce thermal fluctuations in cases where the magnetic head described in the embodiments described below is used. On the other hand, when the value of Brxc3x97t exceeds 5 mA, the signal regenerative output obtained when recording is performed at the maximum linear recording density used in a magnetic storage system decreases greatly with respect to isolated pulse output and the signal processing of regenerative output becomes difficult, posing a problem that the probability of occurrence of errors increases.
When the contents of added boron and chromium or the contents of added phosphorus and chromium are increased in order to reduce magnetic interactions, the grain size of the magnetic layer increases with increasing film thickness although medium noise tends to decrease. In order to prevent this, grains were able to be fined in size by forming a magnetic layer with a raised boron or phosphorus content in the upper part of the magnetic layer which is formed on the side opposite to the substrate, and this was effective in reducing the medium noise. Furthermore, the layering of the magnetic layer via a non-magnetic intermediate layer having the same crystal structure as the magnetic layer was also effective in reducing the medium noise.
Also when the boron or phosphorus content is less than 1 atomic %, the grains of a magnetic layer are fined in size by the addition of boron or phosphorus. However, when the boron or phosphorus content is low, the effect of the addition of these elements on the fining of grain size of the magnetic layer made of the cobalt-base alloy which simultaneously contains germanium and chromium is small.
On the other hand, when boron is added in an amount exceeding 14 atomic % to the cobalt-base alloy which simultaneously contains germanium and chromium, a sputtering target became brittle, posing a problem that working was difficult. Therefore, it is preferred that the boron content be not less than 1 but not more than 14% atomic %. A more preferred range of boron content is not less than 2 but not more than 8 atomic %.
A regenerative output change occurred with the lapse of time in a room-temperature, high-humidity environment in a magnetic recording medium formed through the use of an alloy target in which phosphorus was added in an amount exceeding 12 atomic % to the cobalt-base alloy simultaneously containing germanium and chromium. This decrease in output is due to a decrease in the corrosion resistance of a magnetic film. Because decrease in the phosphorus content improves corrosion resistance, it is preferred that the phosphorus content be not less than 1 but not more than 12 atomic %. A more preferred range of phosphorus content is not less than 2 but not more than 8 atomic %.
Also when the chromium content is from 5 to not more than 15 atomic %, the addition of: boron or phosphorus; and germanium promotes the segregation of chromium at the grain boundaries of the magnetic layer. When chromium is added in an amount of not less than 15 atomic %, this effect becomes more remarkable and the addition of chromium is effective in reducing noise. However, when chromium is added in an amount exceeding 28 atomic %, the remanent magnetic flux density decreases, with the result that the regenerative output becomes too small. Therefore, it is preferred that the chromium content be not less than 15 but not more than 28 atomic %.
With respect to the above ranges of content of the additives, when the total of the boron and chromium contents or the total of phosphorus and chromium contents is not less than 20 but not more than 32 atomic %, the segregation effect of chromium is great and sufficient regenerative output is obtained and, therefore, this range of contents is preferred.
Furthermore, when platinum is added to a cobalt alloy containing the above boron, chromium and germanium or when platinum is added to a cobalt alloy containing the above phosphorous, chromium and germanium, coercivity can be improved and thermal fluctuations can be easily reduced. By replacing a part of platinum with germanium, it becomes possible to lower the melting point of a sputtering target and thereby to raise the degree of migration of atoms during the formation of the thin film. The platinum content is effective when it is not less than 2 atomic %, and the second object can be achieved when the platinum content is up to 50 atomic %. However, when the platinum content is higher than 14 atomic %, the overwriting characteristic during high-frequency recording is deteriorated. When the platinum content is not less than 2 but not more than 14 atomic %, the first and second objects of the invention are simultaneously achieved and mediums obtained in the first and second objects can be used as mediums for achieving the third object. The content of platinum capable of being added as a magnetic recording medium for a magnetic storage system with a relatively high linear recording density and low writing frequency is not less than 2 but not more than 22 atomic %. The overwriting characteristic was deteriorated when the platinum content exceeded 22 atomic %.
When the germanium content is raised, the segregation of Cr is promoted and the addition of germanium becomes very effective in reducing medium noise, and coercivity can be maintained at a high level even when the platinum content is lowered. Therefore, it is preferred that the total of the germanium and platinum contents be in the range of 17 to about 24 atomic %. The effect of the addition of germanium on the above promotion of segregation of Cr and the remarkable reduction in medium noise was observed when the germanium content was not less than 5 atomic %. On the other hand, when the germanium content exceeded 22 atomic %, the yield of target working decreased. Therefore, it is preferred that the germanium content be not less than 5 to not more than 22 atomic %.
With respect the regeneration noise of signals recorded at a high density exceeding 500 kFCI (kilo flux change per inch) for which a high density recording exceeding 20 Gb/inch2 was supposed, an examination was performed regarding the dependence on the contents of: the boron or phosphorus; and chromium contained in the above magnetic layer. As a result, when the total of the contents of: the boron or phosphorus; and chromium contained in the above magnetic layer was reduced to less than 20 atomic %, the regeneration noise of signals recorded at 550 kFCI and 600 kFCI became large in both cases in comparison with other case where the total of the contents of: the boron or phosphorus; and chromium contained in the above magnetic layer was not less than 20 atomic %. Thus, this was unfavorable for reducing noise. Further, when the total of: the boron or phosphorus content; and the chromium content exceeds 32 atomic %, the absolute value of regenerative output becomes small. From the standpoint of regenerative output, therefore, it is preferred that the total of the boron or phosphorus content; and the chromium content be not more than 32 atomic %.